The present invention relates to an exposure apparatus that uses a projection optical system that uses a mirror, to manufacture semiconductors and liquid crystals.
Known as the projection optical system that uses a mirror are an unit magnification mirror optical system for liquid crystals, a reduction catadioptric optical system for excimer-laser exposure, and a multilayer catoptric optical system for extreme ultraviolet (“EUV”) exposure.
For example, Japanese Patent Application, Publication No. 11-219900 (corresponding to U.S. Pat. No. 6,359,678) discloses a configuration for correcting image's positional offsets, a projection magnification, etc. in this mirror including projection optical system. This reference adjusts offsets in position and magnification by driving a reticle stage in an optical-axis direction.
Japanese Patent Application, Publication No. 2000-286191 discloses an exposure apparatus that includes a reticle surface deformation mechanism provided on a reticle holder, measures distortion of an image transferred onto a wafer via a projection optical system, drives an absorption pin provided at a portion that generates the distortion on a pattern surface on the reticle held by the reticle holder based on the measurement result, and deforms the reticle in an optical-axis (or a Z-axis) direction.
In general, any vibration in an optical element in an exposure optical system leads to minute shaking at an image position. More specifically and disadvantageously, vibrations that move a surface in an optical-axis direction lead mainly to vibrations at a focus position, and vibrations that incline the surface leads to an image position in a wafer surface.
A lens and a mirror have different amounts of the image vibrations, although the offset depends upon conditions. For simplicity, a description will be given of a comparison between one mirror and one single thin lens. When the mirror or the single lens offsets by a minute amount in a direction perpendicular to the optical axis, an image's positional offset is approximately the same as the offset of the mirror or single lens. On the other hand, when the mirror or single lens inclines by a fine angle ε around a surface vertex, and an exit angle of light incident upon the surface vertex offsets by 2ε for the mirror but almost zero for the lens before and after the inclination.
When one mirror or a single lens that is used for almost unit magnification imaging changes its position by a fine amount Δz in the optical-axis direction, the image-point position offsets by a fine amount 2Δz in the optical-axis direction for the mirror but the offset is approximately zero in the optical-axis direction for the lens.
Thus, the optical system that includes a mirror produces the image's positional offset, which would not occur in a lens due to self-corrections, although the offset depends upon conditions. In particular, when there are mixed vibrations that move a surface in the optical-axis direction and that incline the surface around the vertex, the mirror produces more remarkable image's offsets than the lens.
The image's vibrations caused by the mirror's vibrations are negligible in a conventional exposure mirror optical system, because of a relatively large critical dimension of a pattern to be exposed. However, a catoptric optical system, such as one for the EUV light, exposes a pattern with a much smaller critical dimension than the conventional one, and the smaller critical dimension enhances influence of the vibrations, preventing an improved resolution.
As a solution for this problem, a proposal by Japanese Patent Application, Publication No. 11-219900 requires driving of the reticle stage in scan and optical-axis directions and thus possibly results in vibrations of the reticle stage itself. In addition, when the vibrations propagate to an optical system, such as a mirror, the optical performance deteriorates. On the other hand, a method disclosed in Japanese Patent Application, Publication No. 2000-286191 mechanically deforms the reticle, causing distortions in the reticle. In addition, it is difficult to deform the reticle into a desired shape and to precisely project an image of a desired shape at a desired magnification.